1. Field of the Invention
The present invention relates to a radio equipment that can change antenna directivity on real time basis as well as to a Doppler frequency estimating circuit used therefor. More specifically, the present invention relates to a radio equipment used in an adaptive array radio base station, and to a Doppler frequency estimating circuit used therefor.
2. Description of the Background Art
Recently, various methods of transmission channel allocation have been proposed to realize effective use of frequency, in a mobile communication system, of which some have been practically implemented.
FIG. 15 shows an arrangement of channels in various communication systems including frequency division multiple access (FDMA), time division multiple access (TDMA) and path division multiple access (PDMA).
Referring to FIG. 15, FDMA, TDMA and PDMA will be briefly described. FIG. 15(A) represents FDMA in which analog signals of users 1 to 4 are subjected to frequency division and transmitted over radio waves of different frequencies f1 to f4, and the signals of respective users 1 to 4 are separated by frequency filters.
In TDMA shown in FIG. 15(B), digitized signals of respective users are transmitted over the radio waves having different frequencies f1 to f4 and time-divided time slot by time slot (time slot: a prescribed time period), and the signals of respective users are separated by the frequency filters and time-synchronization between a base station and mobile terminals of respective users.
Recently, PDMA method has been proposed to improve efficiency of use of radio frequency, as portable telephones have come to be widely used. In the PDMA method, one time slot of one frequency is spatially divided to enable transmission of data of a plurality of users, as shown in FIG. 15(C). In the PDMA, signals of respective users are separated by the frequency filters, the time synchronization between the base station and the mobile terminals of respective users, and a mutual interference eliminating apparatus such as an adaptive array.
The operation principle of such an adaptive array radio base station is described in the following literature, for example:
B. Widrow, et al.: “Adaptive Antenna Systems”, Proc. IEEE, vol.55, No.12, pp.2143-2159 (December 1967).
S. P. Applebaum: “Adaptive Arrays”, IEEE Trans. Antennas & Propag., vol.AP-24, No.5, pp.585-598 (September 1976).
O. L. Frost, III: “Adaptive Least Squares Optimization Subject to Linear Equality Constraints”, SEL-70-055, Technical Report, No.6796-2, Information System Lab., Stanford Univ. (August 1970).
B. Widrow and S. D. Stearns: “Adaptive Signal Processing”, Prentice-Hall, Englewood Cliffs (1985).
R. A. Monzingo and T. W. Miler: “Introduction to Adaptive Arrays”, John Wiley & Sons, New York (1980).
J. E. Hudson: “Adaptive Array Principles”, Peter Peregrinus Ltd., London (1981).
R. T. Compton, Jr.: “Adaptive Antennas—Concepts and Performance”, Prentice-Hall, Englewood Cliffs (1988).
E. Nicolau and D. Zaharia: “Adaptive Arrays”, Elsevier, Amsterdam (1989).
FIG. 16 is a model diagram conceptually showing the operation principle of such an adaptive array radio base station. Referring to FIG. 16, an adaptive array radio base station 1 includes an array antenna 2 formed by n antennas #1, #2, #3, . . . , #n, and a first diagonal line area 3 shows a range in which radio waves from the array antenna 2 can be received. A second diagonal line area 7 shows a range in which radio waves from adjacent another radio base station 6 can be received.
In the area 3, the adaptive array radio base station 1 transmits/receives a radio signal to/from a portable telephone 4 forming a terminal of a user A (arrow 5). In the area 7, the radio base station 6 transmits/receives a radio signal to/from a portable telephone 8 forming a terminal of another user B (arrow 9).
When the radio signal for the portable telephone 4 of the user A happens to be equal in frequency to the radio signal for the portable telephone 8 of the user B, it follows that the radio signal from the portable telephone 8 of the user B serves as an unnecessary interference signal in the area 3 depending on the position of the user B, to disadvantageously mix into the radio signal transmitted between the portable telephone 4 of the user A and the adaptive array radio base station 1.
In this case, it follows that the adaptive array radio base station 1 receiving the mixed radio signals from both users A and B in the aforementioned manner outputs the signals from the users A and B in a mixed state unless some necessary processing is performed, to disadvantageously hinder communication with the regular user A.
[Configuration and Operation of Conventional Adaptive Array Antenna]
In order to eliminate the signal from the user B from the output signal, the adaptive array radio base station 1 performs the following processing. FIG. 17 is a schematic block diagram showing the structure of the adaptive array radio base station 1.
Assuming that A(t) represents the signal from the user A and B(t) represents the signal from the user B, a signal x1(t) received in the first antenna #1 forming the array antenna 2 shown in FIG. 16 is expressed as follows:x1(t)=a1×A(t)+b1×B(t)where a1 and b1 represent factors changing in real time, as described later.
A signal x2(t) received in the second antenna #2 is expressed as follows:x2(t)=a2×A(t)+b2×B(t)where a2 and b2 also represent factors changing in real time.
A signal x3(t) received in the third antenna #3 is expressed as follows:x3(t)=a3×A(t)+b3×B(t)where a3 and b3 also represent factors changing in real time.
Similarly, a signal xn(t) received in the n-th antenna #n is expressed as follows:xn(t)=an×A(t)+bn×B(t)where an and bn also represent factors changing in real time.
The above factors a1, a2, a3, . . . , an show that the antennas #1, #2, #3, . . . , #n forming the array antenna 2 are different in receiving strength from each other with respect to the radio signal from the user A since the relative positions of the antennas #1, #2, #3, . . . , #n are different from each other (the antennas #1, #2, #3, . . . , #n are arranged at intervals about five times the wavelength of the radio signal, i.e., about 1 m, from each other).
The above factors b1, b2, b3, . . . , bn also show that the antennas #1, #2, #3, . . . , #n are different in receiving strength from each other with respect to the radio signal from the user B. The users A and B are moving and hence these factors a1, a2, a3, . . . , an and b1, b2, b3, . . . , bn change in real time.
The signals x1(t), x2(t), x3(t), . . . , xn(t) received in the respective antennas #1, #2, #3, . . . , #n are input to a receiving unit 1R forming the adaptive array radio base station 1 through corresponding switches 10-1, 10-2, 10-3, . . . , 10-n respectively so that the received signals are supplied to a weight vector control unit 11 and to one inputs of corresponding multipliers 12-1, 12-2, 12-3, . . . , 12-n respectively.
Weights w1, w2, w3, . . . , wn for the signals x1(t), x2(t), x3(t), . . . , xn(t) received in the antennas #1, #2, #3, . . . , #n are applied from the weight vector control unit 11 to other inputs of these multipliers 12-1, 12-2, 12-3, . . . , 12-n respectively. The weight vector control unit 11 calculates these weights w1, w2, w3, . . . , wn in real time, as described later.
Therefore, the signal x1(t) received in the antenna #1 is converted to w1×(a1A(t)+b1B(t)) through the multiplier 12-1, the signal x2(t) received in the antenna #2 is converted to w2×(a2A(t)+b2B(t)) through the multiplier 12-2, the signal x3(t) received in the antenna #3 is converted to w3×(a3A(t)+b3B(t)) through the multiplier 12-3, and the signal xn(t) received in the antenna #n is converted to wn×(anA(t)+bnB(t)) through the multiplier 12-n.
An adder 13 adds the outputs of these multipliers 12-1, 12-2, 12-3, . . . , 12-n, and outputs the following signal:
w1(a1A(t)+b1B(t))+w2(a2A(t)+b2B(t))+3(a3A(t)+b3B(t))+ . . . +wn(anA(t)+bnB(t))
This expression is classified into terms related to the signals A(t) and B(t) respectively as follows:
(w1a1+w2a2+w3a3+ . . . + wnan)A(t)+(w1b1+w2b2+w3b3+ . . . +wnbn)B(t)
As described later, the adaptive array radio base station 1 identifies the users A and B and calculates the aforementioned weights w1, w2, w3, . . . , wn to be capable of extracting only the signal from the desired user. Referring to FIG. 17, for example, the weight vector control unit 11 regards the factors a1, a2, a3, . . . , an and b1, b2, b3, . . . , bn as constants and calculates the weights w1, w2, w3, . . . , wn so that the factors of the signals A(t) and B(t) are 1 and 0 as a whole respectively, in order to extract only the signal A(t) from the intended user A for communication.
In other words, the weight vector control unit 11 solves the following simultaneous linear equations, thereby calculating the weights w1, w2, w3, . . . , wn on real time basis so that the factors of the signals A(t) and B(t) are 1 and 0 respectively:w1a1+w2a2+w3a3+ . . . +wnan=1w1b1+w2b2+w3b3+ . . . +wnbn=0
The method of solving the above simultaneous linear equations, not described in this specification, is known as described in the aforementioned literature and already put into practice in an actual adaptive array radio base station.
When setting the weights w1, w2, w3, . . . , wn in the aforementioned manner, the adder 13 outputs the following signal:output signal=1×A(t)+0×B(t)=A(t)
[User Identification, Training Signal]
The aforementioned users A and B are identified as follows:
FIG. 18 is a schematic diagram showing the frame structure of a radio signal for a portable telephone set. The radio signal for the portable telephone set is roughly formed by a preamble consisting of a signal sequence known to the radio base station and data (sound etc.) consisting of a signal sequence unknown to the radio base station.
The signal sequence of the preamble includes a signal sequence of information for recognizing whether or not the user is a desired user for making communication with the radio base station. The weight vector control unit 11 (FIG. 17) of the adaptive array radio base station 1 compares a training signal corresponding to the user A fetched from a memory 14 with the received signal sequence and performs weight vector control (decision of weights) for extracting a signal apparently including the signal sequence corresponding to the user A. The adaptive array radio base station 1 outputs the signal from the user A extracted in the aforementioned manner as an output signal SRX(t).
Referring again to FIG. 17, an external input signal STX(t) is input to a transmission unit 1T forming the adaptive array radio base station 1 and supplied to one inputs of multipliers 15-1, 15-2, 15-3, . . . , 15-n. The weights w1, w2, w3, . . . , wn previously calculated by the weight vector control unit 11 on the basis of the received signal are copied and applied to other inputs of these multipliers 15-1, 15-2, 15-3, . . . , 15-n respectively.
The input signals STX(t) weighted by these multipliers 15-1, 15-2, 15-3, . . . , 15-n are sent to the corresponding antennas #1, #2, #3, . . . , #n through corresponding switches 10-1, 10-2, 10-3, . . . , 10-n respectively, and transmitted into the area 3 shown in FIG. 16.
The signal transmitted through the same array antenna 2 as that in receiving is weighted for the target user A similarly to the received signal, and hence the portable telephone set 4 of the user A receives the transmitted radio signal as if the signal has directivity to the user A. FIG. 19 images such transfer of a radio signal between the user A and the adaptive array radio base station 1. Imaged is such a state that the adaptive array radio base station 1 transmits the radio signal with directivity toward the target portable telephone set 4 of the user A as shown in a virtual area 3a in FIG. 19 in contrast with the area 3 of FIG. 16 showing the range actually receiving radio waves.
As described above, in the PDMA method, a technique is necessary to remove co-channel interference. In this point, an adaptive array that places nulls on the interfering waves adaptively is an effective means, as it can effectively suppress the interfering wave even when the level of the interfering wave is higher than the level of the desired wave.
When an adaptive array is used for a base station, it becomes possible not only to remove interference at the time of reception but also to reduce unnecessary radiation at the time of transmission.
At this time, an array pattern at the time of transmission may be an array pattern for reception, or the array pattern may be newly generated based on a result of incoming direction estimation, for example. The latter method is applicable no matter whether FDD (Frequency Division Duplex) or TDD (Time Division Duplex) is used. It requires, however, a complicated process. When the former approach is to be used while FDD is utilized, modification of the array arrangement or weight becomes necessary, as the array patterns for transmission and reception are different. Therefore, generally, application is on the premise that TDD is utilized, and in an environment where external slots are continuous, satisfactory characteristic has been ensured.
As described above, in the TDD/PDMA method using an adaptive array in the base station, when an array pattern (weight vector pattern) obtained for the up link is used for the down link, transmission directivity may possibly be degraded in the down link because of time difference between the up and down links, assuming a dynamic Rayleigh propagation degree with angular spread.
More specifically, there is a time interval from transmission of the radiowave from a user terminal to the base station through the up link until radiowave is emitted from the base station to the user terminal through the down link. Therefore, if the speed of movement of the user terminal is not negligible, transmission directivity degrades because of the difference between the direction of radiowave emission from the base station and the actual direction of the user terminal.
As a method of estimating weight for the down link considering such a variation in the propagation path, a method of estimating a transmission response vector of the down link by extrapolation utilizing a reception response vector obtained in the up link has been proposed.
When there is an estimation error in the reception response vector estimated for the up link because of a noise in the reception signal or sampling error, there would be an error in the result of extrapolation, making it difficult to correctly estimate the transmission response vector for the down link, and hence, it becomes impossible to realize satisfactory control of transmission directivity.
Now, propagation environment of the propagation path is represented by variation of reception coefficient of the propagation path, that is, degree of fading. The degree of fading is represented by a so-called Doppler frequency (FD) as a physical amount.
More specifically, dependent on the degree of fading on the propagation path, there would be an error in the result of extrapolation. Therefore, in order to prevent extrapolation error, it is necessary to know the degree of fading of the propagation path, that is, to know the Doppler frequency.
A method has been proposed as disclosed, for example, in Japanese Patent Laying-Open No. 7-162360, in which the degree of fading is estimated by finding a correlation value of reference signals included in reception signals received preceding and succeeding in time with respect to each other. In the conventional method, however, the correlation value is calculated using reference signals that are included in the reception signals themselves, which means that there are much interfering components, making correct estimation difficult.
Further, the timing of the reference signals is fixed. Therefore, it was impossible to calculate the correlation value at an arbitrarily timing, and therefore, the calculating process was not flexible.
Meanwhile, the method of estimating Doppler frequency of the propagation path for each user terminal separated by the adaptive array processing has not yet been developed.